The present invention relates to a charge/discharge control circuit which can control electrical charge or discharge of a secondary cell by turning a switch circuit "on" or "off" and a charging type power-supply unit that utilizes the charge/discharge control circuit.
Apower-supply unit whose circuit block diagram is illustrated in FIG. 2 is known as a conventional charging type power-supply unit that is composed of a secondary cell. This structure is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 4-75430, "Power-Supply Unit Of Electrical Charge Type". That is, a secondary cell 101 is connected through a switch circuit 103 to a -VO terminal 105 or +VO terminal 104 which is an external terminal. Further, a charge/discharge control circuit 102 is connected in parallel with the secondary cell 101. This charge/discharge control circuit 102 has a function of detecting the voltage of the secondary cell 101. Either in a case where the secondary cell 101 is in a state of overcharge (a state where the cell voltage value larger than a prescribed voltage value. This state is hereinafter referred to as "an overcharge protection state") or in a case where the secondary cell 101 is in a state of over-discharge (a state where the cell voltage is smaller than a prescribed voltage value. This state is hereinafter referred to as "an over-discharge protection state"), a signal is output from the charge/discharge control circuit 102 so as to turn the switch circuit 103 "off". By stopping the electrical discharge when the voltage of the +VO terminal 104 has reached a certain level of voltage, it is possible to limit the current that flows through the switch 103. That is, when an excessive amount of current is plowing, it is possible to stop the electrical discharge for a (control of excess current). This state is hereinafter referred to as "an excess current protection state". Between the +VO terminal 104 and -VO terminal 105 there is connected an electrical charger 108 for electrically charging the secondary cell 101 or a load 109 that uses the energy of the secondary cell 101.
As another example of the conventional charging type power-supply unit that is composed of a second cell, there is also known a power-supply unit such as that illustrated in a circuit block diagram of FIG. 3. This circuit is one wherein the switch circuit 103 illustrated in FIG. 2 is connected in series with a negative electrode 111.
However, the charge/discharge control circuit that has been constructed as mentioned above has the following drawbacks when a load is connected in the electrical overcharge protection state.
In general, the switch circuit 103 uses two FETs (Field Effect Transistor). As another embodiment wherein this switch circuit is used, there is also known a power-supply unit such as that illustrated in a circuit block diagram of FIG. 4. In the embodiment of FIG. 4, the switch circuit 103 is composed of two FETs, i.e., FET-A 112 and FET-B 113.
In the electrical over-discharge state, the charge/discharge control circuit 102 operates so as to turn the FET-A 112 "off" and, in the electrical overcharge state, operates so as to turn the FET-B 113 "off". For this reason, a signal line for controlling the switch circuit is divided into two parts 107A and 107B. Also, in the excess current state, the control circuit operates so as to turn the FET-A 112 "off".
When, in such a circuit, the load is connected to the cell which is in a state of electrical overcharge as illustrated in FIG. 5, since the FET-B 113 is "off", the discharge current flows through a parasitic diode. For this reason, even when the current that is consumed by the load is small, the voltage of the -VO terminal 105 rises necessarily by the forward voltage Vf of the parasitic diode. Since the representative value of the parasitic diode Vf existing in the FET-B 113 is approximately 0.6 V, the voltage of the -VO terminal 105 becomes higher than the excess current detection voltage. Since when the excess current detection voltage is lower than Vf the connecting of the load in the electrical overcharge state results in the state of electrical overcharge and excess current, both the FET-A 112 and the FET-B 113 go "off", with the result that while the electrical charge is stopped, the electrical discharge made with respect to the load is also interrupted. This indicates that while on one hand the cell voltage is in a state of electrical overcharge and therefore is maintained to have an appreciably high level of voltage, on the other hand electrical discharge becomes disabled immediately after the load has been connected to the cell. Since after the electrical charge is effected the electrical discharge is disabled, the secondary cell completely malfunctions.
When the excess current detection voltage is set to be at a higher value, it results that the current that flows through the FETs becomes large in magnitude with the result that the function of the excess current protection ceases to be performed.